Conventional obstacle detection devices for a vehicle detect an obstacle around the vehicle by means of a distance sensor and notify the driver in the vehicle of the obstacle. One of the obstacle detection devices (hereafter a first device) changes methods for the notification according to the distance to the obstacle. For example, suppose that the distance between the vehicle and the obstacle varies as shown in graphs (a) and (b) in FIG. 13. The graph (a) shows the distance in terms of time and the graph (b) shows velocity of the obstacle relative to the vehicle in terms of time. If the distance is getting shorter, the relative velocity is positive.
In this case, the first device changes the methods for the notification according to the distance when the distance is below a threshold DT1, as shown in a graph (c) in FIG. 13. The graph (c) in FIG. 13 shows intensity of the notification by the first device in terms of time. For example, the first device produces discrete sounds at long intervals when the distance is long, shortens the intervals as the distance gets shorter and produces continuous sounds when the distance is below ten centimeters. Thus, the driver can recognize the distance from the sounds.
However, as seen in the graph (c), the first device makes the notification even if the distance is getting longer. This sometimes bothers the driver.
Another obstacle detection device in JP-H03-135783A (hereafter a second device) operates in at least two stages. At one of the stages where the obstacle is at a short distance, the second device always makes a notification. At the other stage where the obstacle is at a long distance, the second device makes the notification only if the obstacle is getting closer. For example, as shown in a graph (d), which shows intensity of the notification by the second device in terms of time, the second device always makes the notification if the distance is shorter than a threshold DT2. When the distance is longer than the threshold DT2, it makes the notification only if the distance is getting shorter, as shown in a dashed square in the graph (d). Thus, even when the distance is shorter than the threshold DT1, the notification is not made while the distance is getting longer and the distance is longer than the threshold DT2. This reduces the possibility of bothering the driver.
Further obstacle detection device (hereafter a third device) makes the notification by sound just once when it detects the obstacle at a long distance, subsequently continues the notification by only displaying an warning light on a screen and makes the notification by sound again when the obstacle comes to a close position. For example, as shown in a graph (e), which shows intensity of the notification by the third device in terms of time, the third device makes the notification by sound just once when it detects the obstacle at a distance longer than the threshold DT2, and always makes the notification by sound while the distance is shorter than the threshold DT2. Therefore, the third device does not make the notification by sound in a period within the dashed square in the graph (e) in FIG. 13.
However, the second device always makes the notification if the distance is between the thresholds DT1 and DT2 and the obstacle is getting closer. Therefore, the second device makes the notification even if the obstacle is approaching the vehicle slowly. Thus, the second device still has the possibility of bothering the driver.
On the other hand, once the third device first notifies the driver of the obstacle at a long distance by sound, it never makes the notification by sound again until the distance becomes shorter than the threshold DT2. Therefore, even if the obstacle is approaching the vehicle rapidly after the first notification, the third device may fail to notify the driver of such a situation.